1. Field of the Invention
The present invention relates to an X-ray CT (computed tomography) apparatus and a data detecting method of X-ray CT apparatus reconstructing a diagnostic image of an object using X-ray detection data acquired by exposing an X-ray to the object, and more particularly, to an X-ray CT apparatus and a data detecting method of X-ray CT apparatus having a plurality of tubes.
2. Description of the Related Art
An X-ray CT apparatus reconstructs a diagnostic image of an object by applying an X-ray from an X-ray tube to the object, acquiring X-ray detection data by an X-ray detector, and subjecting the acquired data to image reconstruction processing. Half reconstruction is one method for reconstructing a diagnostic image with the X-ray CT apparatus. In contrast to normal image reconstruction processing that reconstructs a diagnostic image by detecting data on an object over a range of 360°, half reconstruction reconstructs a diagnostic image by generating one image on the basis of projection data for an angle less than 360° (in general, projection data for the sum of 180° and the fan angle).
Half reconstruction is frequently used to obtain an image of a part, such as the heart, which moves locally, because of its capabilities of reconstructing an image from projection data for a narrow angle range and obtaining an image with high time resolution. In particular, when an image of the heart is obtained by half reconstruction using X-ray detection data for an angle less than 360° acquired in synchronization with an electrocardiogram (ECG), the influence of the heartbeats on the image can be reduced.
FIG. 10 is a conceptual diagram explaining the method for acquiring data for half reconstruction in synchronization with an electrocardiogram by the conventional X-ray CT apparatus.
As shown in FIG. 10, an X-ray is applied from a tube #1 to an object, and half data Dh is acquired by an X-ray detector during the heartbeats shown in an ECG signal. The angle range of the half data Dh in FIG. 10 is set in the range of approximately 210° to 240°, for example, at 240°.
When the number of heartbeats per minute is 120 bpm, the length tb of one heartbeat is 0.5 sec, and therefore, high corresponding time resolution is required in order to acquire data within one heart beat. In normal cases, however, it is often difficult to sufficiently acquire half data within one heartbeat. Accordingly, a method of acquiring half data in divided segments within a plurality of heartbeats has been proposed.
FIG. 11 is a conceptual diagram explaining the method for acquiring data for half reconstruction from divided segments in synchronization with an electrocardiogram by the conventional X-ray CT apparatus.
In the proposed method, while a tube is helically moved around an object, as shown by a tube locus p in FIG. 11, a first segment S1 of half data is acquired within a first heartbeat B1, and second and third segments S2 and S3 of the half data are acquired within second and third heartbeats B2 and B3 respectively. By data acquisition within three heartbeats, half data for approximately 240° on a reconstruction plane Y can be obtained.
On the other hand, a multi-tube X-ray CT apparatus have been proposed as an attempt to increase the time resolution. In the multi-tube X-ray CT apparatus, X-rays are emitted from a plurality of tubes to an object, and are detected by X-ray detectors arranged opposite to the corresponding tubes.
One of the multi-tube X-ray CT apparatuses is a three-tube X-ray CT apparatus including three tubes. In the proposed three-tube X-ray CT apparatus, three pairs of a tube and a detector are equally spaced 120° apart (see, for example, Japanese Patent Application (Laid-Open) No. H5-168616 or Japanese Patent Application (Laid-Open) No. 2001-346791). The three-tube X-ray CT apparatus enables 360° data to be acquired by rotating each pair 120°. For this reason, ideally, it is expected to acquire data in one third of the time taken for an X-ray CT apparatus including only one tube to acquire data.
FIG. 12 is a conceptual diagram explaining the method for acquiring data by the conventional proposed three-tube X-ray CT apparatus.
When three tubes #1, #2, and #3 are equally spaced 120° apart in a state St01 in FIG. 12, 360° data can be acquired by rotating the tubes #1, #2, and #3 120° into a state St02.
Therefore, when rotation speed of the tubes is 0.3 sec/rot, the time needed to acquire 360° data is 120/360×0.3=0.1 (sec), which is one third of the time taken when one tube is used.
FIG. 13 is a conceptual diagram showing data acquired by the conventional proposed three-tube X-ray CT apparatus.
In FIG. 13, the ordinate indicates the data acquisition range expressed as the angle of application of X-rays to an object, and the abscissa indicates the used channels (Ch) of the X-ray detectors. As shown in FIG. 13, X-ray detectors #1′, #2′, and #3′ opposing the three corresponding tubes #1, #2, and #3 acquire different data for each 120°. The X-ray detectors #1′, #2′, and #3′ are equivalent in terms of the number of channels of detecting elements provided therein, and X-rays are detected in all the channels.
As a result, the equivalent number of data of 120° data D#1′, D#2′ and D#3′ according to the number of the channels is acquired by the X-ray detectors #11, #21 and #3′ respectively, as shown in FIG. 13. That is, 360° data that is proportional to the number of the channels is acquired by the three-tube X-ray CT apparatus.
Such a multi-tube X-ray CT apparatus is considered effective for high-speed scanning from a viewpoint of time resolution.
In the half reconstruction technology, enhancement of the time resolution is important, as described above. However, when half reconstruction is performed by a conventional one-tube X-ray CT apparatus, the time resolution is insufficient, and as a result, it is sometimes difficult to acquire necessary data within one heartbeat. For this reason, data is acquired in segments over a plurality of heartbeats.
Accordingly, the use of a multi-tube X-ray CT apparatus for such data acquisition that requires high time resolution is expected. However, when an image of a local part, such as the heart, is obtained with a multi-tube X-ray CT apparatus including equally spaced tubes, half reconstruction is not used, but full reconstruction is performed to acquire angularly continuous data on imaging.
In the multi-tube X-ray CT apparatus including equally spaced tubes, the tubes need not be rotated 360° because of the number of tubes, and this makes the time resolution higher than when full reconstruction is performed with a one-tube X-ray CT apparatus. On the other hand, it is difficult to use half reconstruction. For this reason, there is a demand to further increase the time resolution on imaging of a local part.
Further, when a plurality of pairs of a tube and an X-ray detector are provided to ensure a sufficient FOV (field of view) on the multi-tube X-ray CT apparatus, the manufacturing cost increases. In general, high time resolution is particularly required mainly in a case in which the FOV is narrow and an image of a local part, such as the heart, is obtained. Therefore, the time resolution is also expected to be increased with a simpler configuration.